The impact of malnutrition in the world is staggering. Malnutrition is thought to directly or indirectly contribute to more than half of all childhood deaths, most of them related to heightened susceptibility to infection. Visceral leishmaniasis (VL), caused by the protozoan Leishmania donovani, is second only to malaria in global mortality among the tropical parasitic diseases. While most people who are infected with Leishmania develop only an asymptomatic latent infection, malnourished individuals have a greatly increased risk for the development of progressive life-threatening VL. Because multiple nutrient deficiencies probably act in concert to impair host defense in human malnutrition, we established a murine model of polynutrient deficiency (PND; deficient in protein, energy, zinc and iron) that closely mimicked moderate acute malnutrition seen in children in resource-poor regions of the world. In this model we recapitulated the epidemiological observations that malnutrition is a risk factor for VL by demonstrating that PND led to a dramatic increase in early dissemination following cutaneous infection with L. donovani. Strikingly, we found that the early visceralization of L. donovani in PND mice was not related to impaired parasite killing, but due to a failure of the skin-draining lymph node (SDLN) to act as a barrier to dissemination. This loss of SDLN barrier function was associated with a significant reduction in the numbers of dendritic cells (DCs) and macrophages, thus leading to an overall reduction in phagocytic capacity of the SDLN. The proposed exploratory studies will fill a significant gap in our understanding of the mechanistic underpinnings of malnutrition-related impairment of SDLN function and defense against Leishmania dissemination. In Specific Aim 1 we will determine the mechanism(s) through which malnutrition leads to a reduced number of macrophages and DCs in the SDLN. Using multicolor flow cytometry, adoptive transfer of mixed fluorescent chimeras, and multi-photon microscopy we will determine if malnutrition reduces SDLN myeloid subpopulations by increasing apoptosis and/or decreasing proliferation, and if reduced migration/retention of myeloid cell in the SDLNs explains the malnutrition-related reduction in SDLN phagocytes. In Specific Aim 2 we will determine the impact of malnutrition on the cutaneous response to L. donovani infection. Using strategies similar to those used in Aim 1 we will determine the effect of malnutrition on the recruitment of myeloid cells and uptake of L. donovani following cutaneous infection. We will determine if chemokine expression has a causal role in the reduced number of myeloid cells in the SDLN and clarify the route of trafficking (lymphatic vs. hematogenous) of parasites from the skin in the malnourished host.